Advances in integrated circuit (IC) technology continue to occur at a rapid rate. On-chip elements and devices are fabricated in smaller sizes, allowing more devices to be fabricated on a chip. Also, chips are now being fabricated that are a few centimeters on a side. IC chip modules such as processors, digital-to-analog (D/A) converters and analog-to-digital (A/D) converters, CMOS active pixel sensors, application specific integrated circuits (ASICs), field programmable logic arrays, digital signal processors, and memory have increased in number and complexity, and generate additional data for on-chip communication. Higher data rates are utilized to keep pace with increased data and processor speeds and larger chip sizes. In some instances, the data rates are not sufficient, thus a demand for a greater numbers of interconnects (i.e., wires or waveguides to couple signals between chip modules) exists.
Technology has advanced to a level at which high speed performance is limited more by interconnect effects than the switching speed of IC semiconductor devices. Data transmitted across interconnects are affected by frequency dispersion, that is, the frequency components of the data signal propagate at different speeds across the interconnect, leading to temporal spreading of the data pulses. ICs are typically densely populated with devices and various elements; therefore it is often not possible to reduce the separation between the chip modules. Consequently, it may not be possible to reduce the length of interconnects between the chip modules. Other requirements such as minimum waveguide dimensions and minimum wire spacings to avoid signal coupling further limit the ability of the designer to achieve closer positioning of the chip modules. The lengths of the interconnects are a significant portion of the chip dimensions and, in some instances, the lengths approach or exceed 2 cm. Consequently, data signals transmitted across the interconnects are subjected to significant dispersion and can experience delays of hundreds of picoseconds or more.
One method for reducing the dispersion and delay of the data signal is based on transmitting optical data pulses across the interconnects. The additional on-chip complexity and increased cost, however, make optical interconnects undesirable for many applications.
Another method is based on mixing a local oscillator signal from an off-chip local oscillator with the data signal to generate an upconverted data signal. The upconverted data signal has frequency components at higher frequencies than the original data signal, consequently interconnect effects are dominated by inductance instead of resistance. After transmission across an interconnect, the upconverted data signal is mixed with the local oscillator signal. The resulting downconverted data signal is amplified and provided to the appropriate chip module. The local oscillator signal distributed to the two mixers requires accurate phase matching. Consequently, attention to path lengths for the local oscillator signal during IC layout and fabrication is critical. Moreover, amplifiers or regenerators may be required to support the distribution of the local oscillator signal across the chip.